Tanning with free formaldehyde and melamine simultaneously



Patented May 17, 1949 TANNING WITH FREE FORMALDEHYDE AND MELAMIN ESIMULTANEOUSLY Wallace Windus, Bristol, Pa., assignor to John K.

Evans & Company, Camden, N. J a' corporation of New Jersey I No Drawing.

Application September 7, 1945,

Serial No. 615,077

6 Claims. (01. 8-94.33)

The present invention relates to improvements in the art of tanninghides and skins, and is particularly applicable to the production ofwhite leather.

The tanning of hides and skins and the modern manufacture of leather maybe divided generally into two broad classes, namely, those which involvetanning with vegetable tannins and those which employ inorganic ormineral substances as tanning agents. Leather produced by the aid ofvegetable tanning agents or tannins, while excellent for many purposes,has a brown color and is not resistant to hot water. On the other hand,mineral tanned leathers have the color which happens to be imparted tothem by the particular tanning agent used, and are characterized byresistance to hot water.

Among the most widely used mineral tanning agents are various salts ofchromium, which impart a shade of blue to the hides or skins. On theother hand, various salts of aluminum and of zirconium have also beenused, these producing a. white leather.

Among the organic tanning agents which are different from the vegetabletanning agents is formaldehyde, which, however, does not produce anentirely satisfactory leather when used as the sole tanning agent.

Recently there have been proposed as tanning agents various organicpolymers or polymerproducing chemicals with which the hides or skinswereimpregnated for the purpose of effecting a result analogous to tanning,with the result that the skins wereconverted into leather. Among thosewhich have been proposed are water-soluble salts of anhydride-vinyl typeof copolymers, and also various water-soluble forms ofmethylolmelamines. It has now been found that a considerable improvementin the production of white leather can be obtained by the simultaneoususe of both melamine and formaldehyde, these two materials being used intheir free state and not in the form of a condensation product.

Accordingly, one of the objects of the invention is a process of tanninghides and skins which comprises subjecting them to the conjoint actionof melamine and of formaldehyde in an aqueous medium.

A further object of the invention is a process of tanning hides andskins which comprises subjecting them to the conjoint action of freemelamine and of free formaldehyde in a slightly acid reacting aqueousmedium.

A further object of the invention is a process of producing leatherwhich comprises immersing or hides.

skins and hides in an aqueous solution of free formaldehyde and of freemelamine for a time suflicient to effect the tanning thereof.

A further object is the production of a melamine formaldehyde-tannedleather or leather containing the direct reaction-products of bothmelamine and of formaldehyde on the skin sub' stance.

In the already mentioned proposals for the use of premade condensationproducts of melamine and formaldehyde, as for example the patent toWilliam Orville Dawson No. 2,316,741,

there is employed a condensation product of melamine and formaldehydewhich is known as the methylolmelamine, which is produced by boiling 1mol of melamine with about 3 /1 mols of an aqueous 30% formaldehyde,having a pH of about 9.0, for about 20 minutes, followed by cooling,separating and .drying the resulting crystals. These crystals arereadily dispersible in hot water. If desired, a, methylolmelaminesolution used directly as a tanning agent may be prepared by heating themelamine and formaldehyde to produce the condensation product, butomitting the step of separating the product from its mother liquor. Inusing these compositions Dawson has proposed that the condensationproduct be applied to skins in a solution of sodium chloride and thateventlually a leather would result.

The present inventor, however, has found that the results obtained bythis methylolmelamine tanning do not produce a material which has a highenough shrink test, which preferably should be of such a nature that theleather would withstand a short actual boiling with water.

Accordingly, the present discovery involves a procedure in which themelamine and formaldehyde are directly and separately added to theaqueous bath in which the skins or hides are being treated, care beingtaken to avoid the formation of a condensation product, but rather toallow both the melamine and the formaldehyde simultaneously to reactwith each other and the skin or hide substance.

In other words, in the present process all of the melamine and all ofthe formaldehyde is available to react with and thereby tan the skins Itwas found that the tanning action in the case of the use of the presentinvention is gradual and mild, partly because of the limited solubilityof the melamine. This is a decided advantage from the standpoint of asmooth grain, as well as of a uniform tannage throughout the entirethickness of the skin or hide. Moreover, the rate of tanning can be morereadily con-- 3 trolled. Thus, it fast tanning is desired, a relativelyslight increase of the temperature of the tanning liquor will effectthis, primarily as a result of the thereby increased solubility of themelamine. It was found that by practicing the present invention, afull5-minute boiling can be obtained, that is to say the leather willnot shrink upon being immersed in'boiling water for 5 minutes. Thisresult is very unusual with tanning agents which yield a white leather,and is superior even to the best white mineral tannage, for which thisboiling test is a practical means of determining the tanning action.

The present process is more economical, not only because the total costof the individual chemicals is lower than that of the premade melamineformaldehyde condensation product, but also because the process can beoperated with a higher degree of emciency. It was found that the entirereaction could be accomplished within a hydrogen-ion concentration rangelying between a pH of 5.5 and 4.2, and that there was no need to acidifythe liquor to a hydrogen-ion concentration of above that equivalent topH 4. Since the hydrogen-ion concentration range is the correct one toallow of the immediate next operation, which is fat-liquoring, two usualsteps in the manufacture of resin-treated leather, namely acidificationand neutralization prior to washing and fat-liquoring, can beeliminated.

It was also found that good tanning could be attained with the use ofless formaldehyde than the three mols theoretically required to formtrimethylolmelamine. Thus, when 1.5 mols of formaldehyde to 1 mol ofmelamine were used, which would theoretically yield a mixture consistingmainly of monomethylolmelamine and dimethylolmelamine, a shrink test of194 F. was

obtained. A shrink test of 190 F. is accepted as an adequate one forwhite leather. Y

It was also found that the strength of the leather is greater when thetanning is so adjusted that the shrink test falls between 190 and 200 F.than when the leather is so fully tanned as to withstand a 5-minuteboiling.

One of the important features of the present invention lies in the factthat it is not necessary to use the conventional neutralizing ordepickling agent so as to depickle the skins prior to the tanning, ashas formerly been the practice, because, contrary to what would havebeen expected, it was found that when 5% or more of melamine (on theweight of the wet skins) is used in the present tanning operation, themelamine itself will act as a depickling agent and also as a bufierbefore there is any significant reaction between the formaldehyde andthe melamine and before the tanning effect becomes manifest. Thus, withabout 10% of melamine, and omitting sodium acetate, a shrink test of 202F. was obtained with a 21-hour tanning operation.

The subjoined examples are given for the purposes of illustrating thepresent invention but are not to be construed as limitationsthereon.

Example I skins and 1 Paprah sheep skin. The weight is that of wet skinsafter pickling, that is, after .4 were added 2% pounds of sodiumchloride and 1 pounds of so-called 60% sodium acetate. The mixture wasagitated for about 5 minutes, in order to allow the material to becomeproperly dissolved and distributed, whereafter the 50- pound pack ofskins was placed therein, and the agitation continued for 5 minutes.Thereafter there were added, separately, 6 pounds of melamine and 12pounds of a Formalin containing 36 to 37% of actual HCHO. In thismixture the skins were then run for 2 hours at a temperature of about 82F. The pH of the liquor at that time was found to be 5.1, and the skinsat the end of the 2-hour period had a shrink test of only 166 F.

There were then added 4 ounces of glacial acetic acid and the skins runfor a period of 3 hours. The pH of the liquor at the end of that timehad dropped to 4.6, but the shrink test of the leather was then already190 to 195 F. The skins were then allowed to remain in the solutionovernight, without agitation. The next morning the run was continued for1 hour, at which time the liquor was found to have a pH of 4.4 and theleather a shrink test of 205 to 212 F. All of these operations wereperformed in the usual tanning drum.

The skins were then washed in the drum for 30 minutes in running waterand were then fatliquored in the usual manner with a mixture of rawneats-foot oil and sulfonated oil, wer then pigmented with clay andtitanium dioxide, and then horsed. The leather was found to beexcellent, very white, full, and well tanned.

Individually the shrink tests were as follows: The pig skin averaged 205F.;' the sheep skin withstood a boil at 212 F. for 3 minutes, and thegoat skin a boil at 212 F. for 5 minutes.

Example II This involved the use of a 50-pound pack wet weight ofpickled skins consisting of 89 bacon rinds, 2 Spanish goat skins, and 2Paprah sheep skins. The were depickled and tanned in a liquor containing3% of salt, 3% of sodium acetate, 10% of melamine and 20% of Formalin,these percentages being based on the drained pickled weight of theskins, the leather being taken as 100%. The ratio of liquid to skins was100%; that is to say, the amount of water was equal in 7 weight to thatof the wet skins. All of the chemicals named were added directly to thewater in the drum and the skins were entered immediately without priordepickling or drumming. After drumming in the solution for 1 hour, thepH thereof was found'to be 5.5, but after 5 hours of additionaldrumming, it had dropped to 4.8; at that time the temperature of theliquor was 84 F., and the shrink test of the leather was 180-185 F. Thedrum was allowed to stand idle during the night. Next morning, the drumwas operated for /2 hour, when the pH of the liquor was determined andfound to be 4.5 while the pH. of the skin on cuts thereof was 4.75 to5.0. The shrink tests on the resulting leather was as follows: Baconrinds, 196 to 201 F.; goat and sheep skins 203 to 205 F. The skins werethen washed, fat-liquored with a mixture of raw and sulfonated oils, andpigmented and horsed they have been horsed up and the liquor allowed todrain therefrom. This 50-pound pack of wet pickled skins was treated asfollows:

To 6 gallons (50 pounds) of water at 77 F.

in the same manner as recited in connection with Example I. The finalshrink test on 4 different samples of the bacon rinds averaged 207 F.,while both the goat and the sheep skins withstood a 1-minute boiling.This is an indication that the amounts of melamine and formaldehyde usedwere somewhat higher than would have been required, and that lessthereof could well have been used.

Example III the skins). All of the chemicals and skins were entered inthe water separately, but at the same time before running the churn. Themelamine was added dry, as in the other examples. After 3 hours runningthe temperature of the liquor was 72 F. and the pH thereof was 5.2.

The shrink test of the leather was 160 F. After setting overnight andrunning for 1 hour in the morning, the pH of the liquor dropped to 4.7and the shrink temperature of the skins averaged 191 F. The tanningaction in this case was not quite as complete as when the work had beenperformed in the drum, undoubtedly because of the lesser amount of,mechanical action. The skins were washed, fat-liquored and pigmented.The leather, however, was excellent, just as it had been in connectionwith Examples 1 and 11.

Some of these skins were then put through the regular process forproducing a black suede, using the ordinary developed dye method. Thenap was satisfactory, and the color was an excellent deep black,superior, if anything, to that of regular tanned leather, probablybecause the melamine formaldehyde tannage acts as a mordant for the dyeon the protein.

Example IV pounds (wet weight) of pickled pig skin fronts were depickledand treated in 200% water, 3% salt, 5% sodium acetate, 8% melamine and16% formaldehyde (36-37% actual HCHO), this r. was only 1 solution ofsalt instead of 3%.

After 15 minutes operation the skins were examined and they were foundto be all right even though a lower amount of salt had been used. Therewas present, however, 5% of sodium acetate. After 6 hours of drumming,the pH of the liquor was 4.9, and the temperature thereof was 86 F. Theshrink test on the skins at that time was 185 F. The drum was thenallowed to set overnight and run again for 3 hours in the morning, atthe end of which time the liquor had a pH of 4.8 and the shrink test ofthe skins averaged 192 F. These skins were washed, fatliquored, andpigmented as usual. The finished leather was good.

Example V 100 pounds wet weight of pickled degreased horse shanks weredepickled and tanned in 100% of water, entered at 105 F., there alsobeing used 5% of salt, 3% of sodium acetate, 6% of melamine, and 12% offormaldehyde, all entered in the drum together with the melamine, addeddry, as usual. After running for 1 hour the pH of the liquor was 4.8 andthe temperature 90 F. It was noticed that the liquor was quite clear,indicating that most of the melamine had by that time become dissolved.The drum was then shut down for 1 hour at the end of which the pH wasmeasured and found to be 4.5, while the shrink temperature of the skinsaveraged 180 R, which is considered very good for such a short time. Thedrum was then started up again. and run for 1 hour, at the end of whichtime the pH had dropped to 4.4, the temperature of the liquor now beingF., while the shrink temperature of the leather produced averaged 203 F.The drum was then allowed to remain inoperative overnight, but run forone-half hour in the morning. A test then made showed that the pH of theliquor was now 4.3 and the temperature was 83 F. Six samples had anaverage shrink temperature of 203 F. which was the same as they had beenon the previous .day after 3 hours of tanning.

This is a good illustration of the marked effect of a slight andpractical increase in the temperature of the liquor on the rate oftanning. Since the liquor became clear, as mentioned above, in a muchshorter time than in previous examples, this is taken as evidence that afaster tanning was caused primarily by the increased solubility of themelamine at the higher temperature. It will thus be apparent that bycontrolling the temperature a simple means for controlling the operationis available.

Example VI 50 pounds (wet weight) of degreased pickled horse shanks weredepickled by running for onehalf hour in of water, 5% salt and 5% sodiumacetate. The pH of the liquor was then 4.7. There were then addedseparately 10% of dry melamine and 10% of Formalin as in the otherexamples, and the drum was then run for 1 hour and then allowed to setfor 1 hour. At the end of that time the pH of the liquor was 5.3, itstemperature 80 F. and the shrink temperature of the leather produced F.The drum was then run one-half hour and allowed to set for 3 hours, atthe end of which time the shrink temperature of the leather had risen toF. After setting overnight the pH of the liquor was 5.2 and the averageshrink temperature of the 1eather194 F. The shanks were washed for 15minutes at 80 F., at the end of which time another shrink test thereonwas made and it was found that it was still 194 F. The skins were thenhorsed, shaved, washed, fat-liquored, and pigmented, again horsed, anddried by pasting. The shrink temperature was then 198 F. It

will be noted that in this example the ratio is 1 mol of melamine to 1mols of formaldehyde both on 100% basis. Theoretically this should yielda. mixture consisting mainly of monomethylolmelamine, anddimethylolmelamine and not the trimethylolmelamine theoreticallypredomimating in accordance with the previous examples. Nevertheless thetanning was found to be entirely adequate for a high quality whiteleather.

The present tanning system may also be employed for combination tanningas when using inorganic metal salts or it may be used as a retan.Examples of these procedures are as folows:

Example VII Melamine to the amount of 1% by the weight of wet skins,that is to say 10 pounds thereof, was added to a 1000-pound pack ofbacon rinds during tanning thereof with a combination of basic zirconiumsulphate and formaldehyde, after the pH of the liquor was brought toabout 4.5. The resulting leather was definitely whiter throughout thefactory operations and produced a better base for finishing in spite ofthe fact 7 that only the regular amounts of clay and titanium dioxidewere used. The shrink test was also a few degrees higher than normal.

Example V III Example TX A 100-gram piece of Bombay goat skin wasdepickled and tanned overnight with 20% of formaldehyde. The pH was 4.5to and the shrink test 182 F. The skin, however, due to the absence ofthe melamine, had the typical empty flaccid feel of the regularformaldehyde tannage.

10 grams of solid melamine were then added to the same liquor which wasstill tanning the same piece of skin, and the tannage continued overanother night. By that time the shrink test had risen to 212 F. and theskin was full and solid just like the melamine formaldehyde tannageattained when both the chemicals were used simultaneously. This is anexcellent example of the effect of the melamine in formaldehyde tannageand is indicative of the fact that both of these reagents, that is tosay, the formaldehyde and the melamine, both react with the hidesubstance as well as with each other to produce the results hereindescribed and claimed.

Ercample X A 100-gram piece of lightweight kip was depickled and tannedin 100% liquor with 5% of salt, 10% of melamine, and 20% of formaldehydein the absence of any neutralizing agent other than the melamine itself.After standing overnight the pH of the liquor was test of the skin was202 F. This indicates that under suitable conditions, melamine itself isadequate as the sole depicklingv agent, and that the tanning reaction issufficiently slow to avoid local action or polymerization externally ofthe leather.

From the above experiments it will be evident that in many instances itwill not be necessary to use the conventional neutralizing or depicklingagent on the pickled skins, provided that at least 5% of melamine (onthe weight of the skins) is employed, since the melamine itself will actas a depickling and buffering agent before there is any significantreaction with the formaldehyde or any tanning takes place.

Thus when using 10% of melamine and omitting the sodium acetate a shrinktest of 202 F. was obtained in about 21 hours. With 5% of melamine, andagain without sodium acetate, the pH of the liquor after 1 hour was 5.3,and after 4 hours 4.3. At the end of 1 hour the center of the skin wasstill quite acid, that is below 4, as would be expected, and even at theend of 4 hours there is still a narrow streak in the center which had apH below 4. However, after standing overnight the pH of the liquor was3.18 and the shrink test was 196 F. which is normal for this amount ofmelamine with the use of a neutralizing agent. It will be appreciatedthat the omission of the neutralizing agent simplifies the procedure andmakes it less expensive.

The present method of tanning can also be combined with other knowntanning methods the present invention are quite unexpected and- 4.9 andthe shrink and while of particular advantage in the manufacture of whiteand light colored leathers, is also suitable for making coloredleathers. No particular theory is advanced to explain the effect of thetanning, but it is evident that both the melamine and the formaldehydework independently of each other on the hide substance and probably formsubstances which then react with each other and possibly also with theremaining hide substance to produce the leather. The effects producedare startlingly better than when precondensed methylolmelamine is used.This may be perhaps occasioned by the fact that methylolmelamine, beingalready a condensation product, has a high molecular weight andtherefore its solution does not have the penetrating power of theseparate melamine and formaldehyde. The conditions under which thereagents are used are very much milder than would suggest the formationof condensation products directly between them. Inasmuch as it was notknown that free melamine when used in conjunction with free formaldehydewould have the hereinabove described excellent tanning action, thereactions and effects obtained in accordance with of a surprisingnature, and can unhesitatingly be qualified as being in the nature of aquite unpredictable and unexpected result.

There is considerable latitude in the temperature at which theoperations may be accomplished and also the relative weights of liquorof the various types of skins employed. These proportions are, however,well within the skill of those familiar with the tanning art.

Accordingly applicant claims as his invention:

1. Process of tanning which comprises submerging a tannable animal skinfor at least three hours in a dilute aqueous solution containing atleast about 5% by weight (on the basis of the wet skin) of melamine insuspension together with an amount of dissolved formaldehyde equivalentto at least 1.5 mols of said melamine, the melamine progressivelypassing into solution as the solution becomes depleted of melamine asthe result of the tanning action.

2. Process of tanning which comprises sub- .iecting animal skins to theaction of an aqueous solution containing at least about 5% of freemelamine on the basis of thewet weight of the skins, as well as morethan sufficient free formaldehyde potentially to react with saidmelamine, both the melamine and the formaldehyde remaining substantiallyunreacted at the beginning of the tanning process, the latter beingcarried out for a period of at least three hours, whereby a leatherhaving a shrink test of at least about F. is obtained.

3. The process of claim 2 in which the solution has a hydrogen ionconcentration of between about pH 4.0 and pH 5.5 and the time is fromabout three hours to about 21 hours.

4. Process of tanning which comprises subjecting pickled animal skins tothe action of an aqueous solution containing at least about 5% on thebasis of the wet weight of the skins of free melamine and more thanenough free formaldehyde potentially to react with said melamine, boththe formaldehyde and the melamine remaining substantially unreacted atthe beginning of the tanning process, most of the free melamine beingundissolved and subject to gradual solution as the ensuing tanningaction consumes the dissolved melamine, the process being continued fora period of at least three hours, whereby leather 9 having a shrink testof at least 190 F. is obtained.

5. Process of tanning which comprises subjecting wet pickled animalskins to the action of about an equal weight of water (as based on theweight oi said wet skins) containing from about 6% to about 20% byweight of therei dissolved Formaiin (of 36-37% actual formaldehydecontent) and from about 1% to about 12% by weight of free melamine, atleast some of which remains undissolved during the early stages of theensuing tanning action, the resulting solution having a hydrogen-ionconcentration of about pH 4.0 to pH 5.5, the action being continued fora period of from about three hours to about 21 hours, whereby alight-colored leather having a shrink test of at least about 190 F. isobtained.

6. Process of retanning chromium-tanned leather without previousde-tanning which comprises subjecting it for about three hours to an goaqueous solution containing about 3% of free melamine and about 6% offree formaldehyde at about 130 F. and at a hydrogen-ion concentration01' about pH 4.5, the melamine and form- 10 aldehyde being,substantially unreacted at the beginning of the retanning Process.

I WALLACE WINDUS.

REFERENCES mm The following references are of record in the file of thispatent:

OTHER REFERENCES Resin Tannages," by J. G. Niedercorn, abstract inJournal Amer. Leather Chem. Assoc. for April 1944, page 149. Availablein Scientific Library of Patent Ofllce.

